Mail processing systems, such as, for example, a mailing machine, often include different modules that automate the processes of producing mail pieces. The typical mailing machine includes a variety of different modules or sub-systems, each of which performs a different task on the mail piece. The mail piece is conveyed downstream utilizing a transport mechanism, such as rollers or a belt, to each of the modules. Such modules could include, for example, a singulating module for separating a stack of mail pieces such that the mail pieces are conveyed one at a time along the transport path, a stripping/moistening module for stripping open the flap of an envelope, and wetting and sealing the glued flap of an envelope, a weighing module for weighing the mail piece, and a metering/printing module for storing postage amounts and applying evidence of postage either directly to the mail piece or to a tape to be applied to the mail piece. The mailing machine is controlled by a central processing unit that executes software stored in memory provided in the mailing machine. The exact configuration of the mailing machine is, of course, particular to the needs of the user.
The metering/printing modules of many current mailing machines utilize ink jet printing technology to print evidence of postage, such as postal indicia that include a 2-D barcode. Ink jet printers are well known in the art. Generally, an ink jet printer includes an array of nozzles (sometimes referred to as orifices), a supply of ink, a plurality of ejection elements (for example, expanding vapor bubble elements or piezoelectric transducer elements) corresponding to the array of nozzles and suitable driver and control electronics for controlling the ejection elements. Typically, the array of nozzles and the ejection elements along with their associated components are referred to as a print head. It is the activation of the ejection elements that causes drops of ink to be expelled from the nozzles. The ink ejected in this manner forms drops which travel along a flight path until they reach a print medium such as a sheet of paper, an envelope or the like. Once they reach the print medium, the drops dry and collectively form a print image. Typically, the ejection elements are selectively activated (energized) or not activated (not energized) to expel or not expel, respectively, drops of ink as relative movement is provided between the print head and the print medium so that a predetermined or desired print image is achieved.
Typically, the array of nozzles is disposed at an angle to the direction of movement of the print media along their respective feed paths. This is done so that the print head will print a denser image than would be obtained if the array of nozzles were disposed in a direction that is perpendicular to the direction of movement of the print media. Because of the physical size of the ejection elements that cause ink to be expelled from the nozzles, they may, in many cases, not be able to be spaced sufficiently close together to produce a clear, dense image when arranged perpendicular to the direction of movement of the print media, and by disposing them at an angle to this direction of movement and energizing the ejection elements in an appropriate sequence, the effect on the printed matter is the same as if the nozzles were to be spaced more closely together.
In addition, it is often the case that a number of so angled arrays of nozzles are utilized in one device and are arranged along an axis that is perpendicular to the direction of movement of the print media. For example, a single printing device may utilize 160 nozzles arranged in 10 arrays of 16 nozzles each, with each array being arranged, in an angled manner, along an axis that is perpendicular to the direction of movement of the print media. As will be appreciated, in such a configuration, the first nozzles from each array will be lined up with one another along the axis, the second nozzles from each array will be lined up with another along the axis, and so on. Furthermore, in such a device, the ejection elements corresponding to each of the first nozzles will be simultaneously activated or not activated, the ejection elements corresponding to each of the second nozzles will be simultaneously activated or not activated, and so on, depending on the image to be printed, under the control of the driver and control electronics of the print head.
The transport mechanism of a mailing machine also typically includes an encoder system that acts a mechanical timer for generating firing pulses for the print head and thus timing the printing operation. The encoder system includes an encoder disk that has a plurality of apertures located around its circumference, a light source and a light detector. As the transport mechanism conveys mail pieces along the mailing machine, it causes the encoder disk to rotate. The encoder disk, the light source and the encoder detector are positioned with respect to one another so that encoder disk causes the light source to be alternately blocked and unblocked as the encoder disk rotates. The transition from blocked to unblocked or vice versa results in a change of state, wherein each change of state from blocked to unblocked will indicate that a firing pulse should be generated. In such a case, the encoder system will generate a signal indicating same. Thus, as will be appreciated, the timing of the printing by the print head is tied to the movement of the mail pieces.
Two measures that customers use to evaluate mailing machines are throughput and print quality. Both of these are important to the overall operational efficiency of the mailing machine. Throughput is generally defined by the number of envelopes that the mailing machine can process over a given period of time (e.g., a number of envelopes per minute or a number of envelopes per hour). A higher rate of throughput lowers the processing cost per envelope by amortizing the cost of the mailing machine over a greater number of envelopes.
In addition, printed image quality of postal indicia is important to ensure that the postal authority promptly delivers the mail pieces and that the customer does not incur any loss of postal finds. To protect the stream of postal revenues, the postal authority is constantly on guard against fraudulent postal indicia. As a result, the postal authority inspects incoming mail pieces to determine whether or not the postal indicia are authentic representations that the postal value indicated has been properly accounted for. To perform this inspection, the postal authority requires high quality printed postal indicia so that the information contained within the postal indicia may be easily read and used to verify the integrity thereof. On the other hand, if postal indicia are poorly printed and the authenticity of thereof cannot be determined, then the associated mail pieces are likely to be returned to the sender. The return of mail pieces causes an interruption of business communications and can result in the customer losing the postal funds associated with the returned mail pieces.
The operating frequency of a print head refers to the frequency, in cycles per unit of time such as a second, at which all of the selected nozzles of an array of nozzles in the print head are sequentially activated, in response to a firing signal or pulse, to produce a desired column of drops to form part of an image. As will be appreciated, the inverse of operating frequency is the time (number of seconds) between firing pulses. Throughput, in terms of transport speed (inches per second), print quality, in terms of print resolution (dots per inch or dpi), and print head operating frequency are related to one another as follows:Operating Frequency=Transport Speed×ResolutionSimilarly, throughput, in terms of transport speed (inches per second), print quality, in terms of print resolution (dots per inch or dpi), and time between firing pulses are related to one another as follows:Time Between Firing Pulses=1/Operating Frequency=1/Transport Speed×Resolution
Thus, in an ideal world, a given, required printing resolution could be maintained as transport speed is increased (to increase throughput) simply by also increasing the operating frequency of the print head. The problem, however, is that ink jet print heads, such as those used with current mailing machines, have a maximum operating frequency and a corresponding minimum time between firing pulses. In other words, for each desired column of drops to be printed by the print head, there is a minimum amount of time that it takes for each of the selected nozzles in the print head to fire (be activated and eject a drop) in response to a firing signal or pulse. The next column of drops to be printed by the print head to form the next part of the image cannot be printed until this time has elapsed. As a result, the maximum operating frequency and corresponding minimum time between firing pulses of a print head limits the speed at which mail pieces may be transported along the mailing machine (the transport speed), and therefore limits throughput.
Thus, given a required resolution and a maximum operating frequency in a particular printing application, such as a particular mailing machine that prints particular indicia, an upper transport speed limit can be determined using the equations provided above. For example, if a mailing machine printing application requires a resolution of one dot per every 0.00333 inches (approximately 300 dpi), and the mailing machine print head has a maximum operating frequency of 17.24 KHz (which corresponds to 58 microseconds between firing pulses), then the maximum transport speed that may be used in the mailing machine is approximately 57.26 inches per second (17,240 cycles per second/300 dpi). The problem, however, is that mailing machine transport mechanisms typically have a transport speed tolerance of ±10%, which means that the transport speed, if set at 57.26 inches per second, could actually range from approximately 63 inches per second to approximately 51.5 inches per second. This is problematic because the printing resolution will be decreased significantly (it will go to half density) during any periods where the maximum transport speed is exceeded. In order to avoid this problem, prior art devices and applications have set the operating transport speed of the mailing machine to some fraction of the calculated maximum transport speed described above such that, given the upper tolerance of the transport mechanism, the actual transport speed will never exceed the maximum transport speed. In the example provided above, the operating transport speed may be set to 50 inches per second, in which case the actual transport speed may go as high as 55 inches per second, which is below the calculated maximum transport speed. This prior art solution, while effective, obviously results in a lower throughput than could be achieved if the calculated maximum transport speed could be used. Thus, there is a need for a system and method for enabling the calculated maximum transport speed of a print head in a particular printing situation to be used without sacrificing resolution of the printed image in the process.